The present invention relates to an automatic phase and amplitude equalizer.
An outline of a multilevel signal or pulse amplitude modulation (PAM) signal transmission system including a transversal type of automatic equalizer in the prior art is shown in FIGS. 1 and 2. In FIG. 1, reference numeral 1 designates a roll-off filter whose transfer function is represented by G(f). Reference numeral 2 designates a low-pass filter for removing undesired out-of-band signals, its transfer function being represented by B(f). The filters 1 and 2 correspond to a transmission line for the multilevel signals. In addition, reference numeral 3 designates a phase synchronizer circuit which regenerates a timing signal (called "identifying timing signal") synchronized with the baud rate frequency f.sub.o of the multilevel (PAM) signal, by extracting from the input multilevel signal a pilot signal contained therein. The timing signal is used for operating the automatic equalizer to be detailed later.
An equivalent base band transfer function X(f) for filters 1 and 2 in FIG. 1 is given by: EQU X(f) = G(f) .multidot. B(f) .multidot. e.sup.j2.pi.f.tau. ( 1)
where .tau. represents an identifying timing point. When an impulse having an amplitude of unity is applied to the filter 1 as an input signal to the transmission line, the output response signal x(t+.tau.) can be obtained from an inverse Fourier transformation of the equivalent base band transfer function X(f), so that it can be represented by the following function of time: EQU x(t+.tau.) = F.sup.-1 [X(f)] (2)
where F.sup.-1 [ ] represents an inverse Fourier transformation.
Now an output response signal x.sub.n at identifying time points is given by EQU x.sub.n = x( nT+.tau.) (3)
where n is an integer and T is a baud rate period (l/f.sub.o).
Here, it is assumed that said identifying timing point .tau. is normalized with respect to T and represented by a symbol, EQU .xi. = .tau./T (4)
and this is called "an identifying timing phase" (also called "a timing phase"). Accordingly, Equation-(1) is represented as follows: EQU X(f) = G(f) .multidot. B(f) .multidot. e.sup.j2.pi.f.xi.T ( 5)
next, a description will be given, made with reference to FIG. 2, of a principle of a transversal type of automatic equalizer for equalizing this x(t+.tau.). FIG. 2 and the phase synchronizer circuit 3 in FIG. 1 correspond to a receiving station. In FIG. 2, reference numeral 21 designates a tapped delay line having tap terminals at a time interval equal to the baud rate period T. Character C.sub.k represents a weight factor of a variable tap weighting circuit 22 (k) provided for each tap, where character k represents a tap number (-Nt&lt;k&lt;Nt and the number of taps is 2Nt+1). Numeral 23 designates a summing amplifier. The summing amplifier 23 sums the multilevel (PAM) signals controlled by the respective tap weighting circuits 22(k), and provides an equalized multilevel (PAM) signal at its output terminal 231. The signal derived from the terminal 231 is fed to a sampler 25. In the sampler 25, the equalized multilevel (PAM) signal is sampled by means of an identifying timing signal fed from a phase shifter 30, and the sampled signal is fed to a slicer 26 and a subtracter 28. The slicer 26 delivers an identified signal of the sampled signal from the output terminal 27. The subtracter 28 compares the outputs of the sampler 25 and of the slicer 26 and generates an error signal corresponding to the difference between them. This error signal is fed to multipliers 24 (k). A comparator 31 supplies a signal representative of the difference between the outputs of the variable tap weighting circuits 24 (-i) and 24 (i) to an integrator 29. The integrated output is supplied to a phase shifter 30 and serves as a control signal for controlling the phase of the regenerated identifying timing signal supplied from the terminal 32.
In the transversal type of automatic equalizer, the signal x(t+.tau.) to be equalized is passed through the delay line 21, and signals obtained at each tap at every time interval T are controlled in the variable weighting circuits 22 (k) for each tap, a total sum of them being delivered as an output. The waveform equalization is thus accomplished by automatically controlling the weight factor {C.sub.k } in response to the output of each multipliers 24 (k). In the multiplier 24 (k), the multiplication of the error signal fed from the subtractor 28 and the multilevel (PAM) signal fed from each tap of the delay line 21 is accomplished. The output signals of the multipliers 24 (k) serve as tap control signals for controlling the respective variable tap weighting circuits 22 (k). By continuously carrying out the aforementioned operations in a closed loop, the phase of the identifying timing signal can be controlled so as to automatically accomplish optimum equalization.
The equalizer shown in FIG. 2, however, has a disadvantage in that the equalizable delay and attenuation ranges are narrow.